When transmitting a video signal of an image from a transmitting device to a display unit, it is customary to transmit, together with pixel data, a horizontal synchronizing signal and a vertical synchronizing signal to the display unit in synchronism with clock pulses. If the display unit comprises a CRT, then the frequency and phase of a sawtooth wave that is required to deflect an electron beam in the CRT are controlled on the basis of the horizontal and vertical synchronizing signals to scan the electron beam on the bulb face of the CRT in exactly the same manner that the image has been scanned in the transmitting device.
Recent years have seen the use of thin panel display units in place of conventional display units in the form of CRTs. The panel display units include a liquid-crystal display unit, a plasma display unit, and an EL display unit among others. The panel display units have X and Y electrodes crossing each other. When a voltage is applied to selected X and Y electrodes, light is emitted from the points of intersection of those X and Y electrodes to which the voltage is applied. To supply a video signal from a transmitting device to such a panel display unit, each bit of pixel data is synchronized with a sampling clock pulse on the transmitting device, and supplied, together with horizontal and vertical synchronizing signals, to the panel display unit.
When the panel display unit receives the supplied video signal, it effects horizontal and vertical synchronization of the received video signal, reproduces a sampling clock signal at the same frequency as that of the sampling clock signal on the transmitting device, synchronizes each bit of pixel data generated from the video signal with a sampling clock pulse, and displays the pixel data on a display screen.
For accurately introducing pixel data bit by bit, it is necessary to generate a sampling clock pulse substantially at the center of each bit pulse of the pixel data. However, only the horizontal and vertical synchronization fails to bring the center of each bit pulse into accurate phase with a sampling clock pulse though the periods of these pulses may be synchronized with each other. For this reason, the pixel data may not accurately be latched. The center of each bit pulse tends to be easily shifted out of phase with a sampling clock pulse in a numerical control system or the like which employs a panel display unit because the panel display unit is usually positioned remotely from a transmitting device in the numerical control system.
Heretofore, it has been customary for the panel display unit to be equipped with a .manual phase control device for shifting the phase of sampling clock pulses stepwise. The manual phase control device is manually adjusted to shift the sampling clock pulses into phase with pixel data.
In some applications where a panel display unit with a manual phase control device is combined with a system in a factory such as a numerical control system, since the system is assembled in installations that may change frequently, the length of the cable which interconnects the transmitting device and the panel display unit may also vary depending on the equipment with which the system associated. Each time the length of the cable which interconnects the transmitting device and the panel display unit varies, the transmitting device and the panel display unit are brought into a different phase relationship, causing the operator to manually adjust the manual phase control device for keeping the transmitting device and the panel display unit in phase with each other. Therefore, the manual phase control device has been complex and inefficient to operate.